The noble idea of the "helpful reply" tag is to save readers time by skipping over the chaff and going straight to the best answers.
Being entirely user-defined, anyone can judge for themselves whether or not a post was helpful and pronounce their judgement, justified or not. Such democracy naturally dilutes the feature's value, but that is the nature of the internet.
Of course, here in the CH nothing is to be taken seriously, so the tags have no real meaning. They are, however, a way to annoy fellow CH denizens, who'll receive an email notification every time you mark one of their replies as "helpful".